Apparatus for manufacturing an optical fiber preform and including a ring for trapping and diffusing light radiation

ABSTRACT

The invention relates to apparatus for manufacturing an optical fiber preform from a tube of vitreous material, the apparatus comprising:  
     means for holding said tube during manufacture of said preform; and  
     heater means supplying said tube with the heat energy required for manufacturing said preform.  
     According to the invention, diffuser means are disposed around at least one of the ends of said tube to trap and diffuse light radiation generated in said tube by said heater means. The invention also applies to apparatus for drawing an optical fiber from a preform.

[0001] The invention relates to apparatus for manufacturing an opticalfiber preform, the apparatus comprising a glassmakers' lathe for holdinga glass tube by means of chucks clamped to the two ends of the tube, gasfeed means for injecting gaseous compounds into the inside of the tubevia a leakproof connection at one end of the tube, and a source of theenergy required for causing the gaseous compounds to be deposited insidethe tube or for collapsing the tube after deposition has been completed.

BACKGROUND OF THE INVENTION

[0002] In such a device, heat energy from the source is transmitted tothe leakproof connection via the corresponding end of the glass tube.This connection has a sealing gasket which is thus raised to a hightemperature while the gaseous compounds are being deposited or while thetube is being collapsed. Unfortunately, it is very important for thegasket to maintain good sealing so as to prevent any risk of the gaseouscompounds being exhausted into the medium surrounding the apparatus, andto avoid any contamination of the deposit by the surrounding medium.

[0003] A known solution to the problem of the leakproof connectionbetween the gas feed means and the end of the tube being overheatedconsists in designing a gasket of a complex shape and in making it outof special materials so as to guarantee that the connection remainssealed at the temperatures generated by a blowtorch. Unfortunately, thatsolution increases the cost of manufacturing a preform.

[0004] Another known solution to the above problem consists in holdingthe glass tube on the lathe by means of a tubular endpiece. In this way,the end of the tube is located at a certain distance from the leakproofconnection and the corresponding chuck which come into contact only withthe tubular endpiece. The inside diameter of the endpiece is slightlygreater than the outside diameter of the glass tube and it is assembledthereto by adhesive acting on an overlap zone where the glass tube isreceived in the tubular endpiece. This establishes a discontinuity whichenables a large part of the heat energy to be dissipated by means of theend section of the glass tube. Nevertheless, that solution alsoincreases cost due to making and assembling the endpiece.

[0005] After a preform has been subjected to an internal depositionmethod, similar problems arise when the preform is collapsed. At thattime, the preform is held at its ends by the chucks. While the preformis being heated by the heater means for the purpose of collapsing it,the thermal radiation generated by the heater means heats the chucksthat are holding the preform, thereby running a risk of the chucksbecoming jammed.

[0006] Finally, similar problems arise when drawing an optical fiberfrom a preform. At that time, the preform is held vertically at one ofits ends by means of a chuck. While the preform is being heated by thefiber-drawing oven, the thermal radiation it generates heats the chuckthat is holding the preform and that also can run the risk of the chuckbecoming jammed.

OBJECTS AND SUMMARY OF THE INVENTION

[0007] An object of the invention is to solve the problem of theleakproof connection between the gas feed unit and the end of the glasstube overheating in apparatus for manufacturing a preform, and to do ina manner that combines effectiveness and low cost.

[0008] Another object of the invention is to solve the problem ofoverheating the means for holding a preform while it is being collapsedor while a fiber is being drawn therefrom.

[0009] To this end, the invention provides apparatus for manufacturingan optical fiber preform from a tube of vitreous material, the apparatuscomprising:

[0010] means for holding said tube during manufacture of said preform;and

[0011] heater means supplying said tube with the heat energy requiredfor manufacturing said preform,

[0012] wherein diffuser means are placed around at least one of the endsof said tube to trap and diffuse light radiation generated in said tubeby said heater means.

[0013] The invention also provides an apparatus for drawing an opticalfiber from a preform, the apparatus comprising:

[0014] means for holding one end of said preform; and

[0015] means for lowering said preform as held by said end through afiber drawing oven;

[0016] wherein diffuser means are disposed around said end of saidpreform to trap and diffuse light radiation generated in said end bysaid fiber drawing oven.

[0017] The heat energy supplied by the heater means propagates along thetube or the preform towards its ends mainly in the form of lightradiation. The heat energy which is thus trapped and diffused by thediffuser means is therefore not transmitted to the leakproof connectionor to the supporting chuck, thereby contributing to reducing the heatingthereof.

[0018] Advantageously, the diffuser means has a refractive index closeto that of the tube. By way of example, it can likewise be made of avitreous material. This makes it possible to obtain good transmission ofthe radiation between the tube and the diffuser means.

[0019] Also advantageously, a liquid film can be inserted between thediffuser means and the tube. The light radiation passes from the tube tothe diffuser means via the liquid film, and the diffuser means diffusesit to the surrounding medium. This increases the amount of lightradiation that is transmitted from the tube to the diffuser means. Theliquid film can have a refractive index that is close to that of thetube and of the diffuser means.

[0020] The invention makes it quick and easy to place the diffuser meanson the glass tube, thereby making savings in equipment costs and inmachine time costs. In addition, the risk of breaking the glass tubewhile the diffuser means is being mounted is considerably reducedcompared with sticking on a tubular endpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other characteristics and advantages of the invention will appearon reading the following description of an embodiment of the inventionas shown by the drawings. In the figures:

[0022]FIG. 1 is a diagram showing apparatus of the invention formanufacturing a preform;

[0023]FIG. 2 is an enlarged view of FIG. 1 showing the diffuser means ofthe invention; and

[0024]FIG. 3 is a diagram showing apparatus of the invention for drawinga fiber from a preform.

MORE DETAILED DESCRIPTION

[0025] A method of manufacturing a preform, e.g. the method known underthe initials MCVD (for modified chemical vapor deposition) isimplemented by means of apparatus comprising, FIG. 1, a glassmakers'lathe 1 which holds a silica-based tube 3 via two chucks 5A and 5Bclamped onto the two ends 3A and 3B of the glass tube 3, and which causeit to rotate, R, about its axis. A gas feeder 7 serves to inject oxygenO₂ charged with the vapors of compounds such as silicon tetrachlorideSiCl₄ and germanium tetrachloride GeCl₄, for example, to be injectedinto the inside of the silica tube 3. A blowtorch 9 heats the silicatube 3 and moves in translation T therealong in the same direction asthe direction in which the gaseous compounds flow. In FIG. 1, thisdirection is from the end 3A clamped in the chuck 5A towards the end 3Bclamped in the chuck 5B. On coming close to the end 3B, the blowtorch 9returns quickly to its starting point to begin a new pass.

[0026] The blowtorch 9 supplies the heat energy required for depositinggaseous compounds inside the glass tube. In the heating zone closest tothe blowtorch 9, the gaseous compounds react, forming particles whichthen deposit on the inside wall of the silica tube 3 downstream from theblowtorch 9 in the gas flow direction. The deposited particles are thenvitrified by the blowtorch 9 as it moves along the silica tube 3.

[0027] At the end of deposition, the silica tube 3 has a claddingprecursor formed by the first-deposited layers and a core precursorformed by the last-deposited layers. The gas feeder 7 is then turned offso as to stop injecting gas. The operation of collapsing the silica tube3 then begins, during which the blowtorch 9 raises the temperature ofthe silica tube 3 so as to soften the silica and cause the tube toshrink. After a “starting” pass, the blowtorch 9 is moved very slowly soas to close up the silica tube in the form of a solid preform, whichpreform may optionally be recharged, i.e. additional silica-based layersmay optionally be deposited thereon.

[0028] Communication between the gas feeder 7 and the silica tube 3takes place at the end 3A of the tube via a leakproof connection 13which includes an O-ring gasket 13A. It is very important for the gasketto be leakproof so as to prevent any risk of the gaseous compounds beingrejected into the medium surrounding the apparatus and conversely anyrisk of the MCVD deposition being contaminated by the surroundingmedium.

[0029] In the invention, diffuser means in the form of a circularlycylindrical ring 15 are placed around the silica tube 3 close to theleakproof connection 13, downstream from the chuck 5A clamped to the end3A of the silica tube 3. As mentioned above, the ring 15 can itself bemade of silica so as to have a refractive index that is substantiallyequal to that of the tube 3 so that the change of index at the interfaceis negligible.

[0030] A liquid is inserted between the glass ring and the tube so as toform a film 17 (see FIG. 2). The heat energy supplied by the blowtorch 9during deposition or during collapsing propagates along the silica tube3 towards its ends 3A and 3B mainly in the form of light radiation. Bymeans of the liquid film 17, this light radiation passes from the tube 3to the ring 15 which diffuses it into the surrounding medium. The heatenergy that is trapped and diffused in this way by the silica ring 15 istherefore not transmitted to the leakproof connection 13, therebycontributing to reducing heating thereof. The presence of the liquidfilm 17 increases the transmission of the light radiation between thetwo parts.

[0031] The liquid film 17 can likewise have a refractive index that issubstantially equal to that of the glass ring 15 and of the glass tube3. As can be seen in FIG. 2, the ring 15 is held in place relative tothe tube 3 by means of two O-rings 19 which also hold the liquid film 17between the ring 15 and the tube 3. By using lip gaskets, mounted inappropriate housings formed in the silica ring 15, it is possible toguarantee constant clearance relative to the silica tube 3. The liquidfilm 17 is distributed uniformly at the interface between the twopieces, thereby further contributing to good transmission of lightradiation.

[0032] In a variant embodiment of the invention, the liquid film 17 isinserted in contact with a surface 15I of the ring 15 which is carefullypolished so as to further increase the transmission of light radiationthrough the interface between the ring and the tube. Provision is alsomade to frost the side faces 15A and 15B and the outside face 15E of thesilica ring 15 so as to increase the diffusion of light radiation in theform of heat into the medium surrounding the apparatus for manufacturingthe preform.

[0033] Provision can also be made to cool the silica ring 15 by means ofa compressed air blower 21 for maintaining the liquid film 17 at atemperature such that its refractive index remains substantiallyconstant and equal to that of the ring 15 and of the tube 3. A deflector23 prevents the cold air delivered by the blower 21 from disturbingheating of the silica tube 3 by the blowtorch 9.

[0034] The invention is also applicable to apparatus for drawing anoptical fiber from a preform. Apparatus of this type comprises (see FIG.3) lowering means 25 for lowering a preform 27 through a fiber drawingoven 9′. The lowering means 25 comprise a blind chuck 5′ which ismounted to move in translation parallel to a fiber drawing axis L so asto displace the preform 27 by means of a glass bar 3′ that is clamped atone end in the chuck 5′ and welded at its other end to the preform 27.

[0035] According to the invention, a glass ring 15′ is placed around theglass bar 3′ and located close to the chuck 5′ to trap and diffuse lightradiation generated in the glass bar 3′ by the fiber drawing oven 9′.

[0036] As mentioned above, the ring 15′ placed around the bar 3′ has arefractive index that is substantially equal to that of the bar 3′ sothat the change of index at the interface is negligible. A liquid isthen inserted between the glass ring and the bar 3′ so as to form a film17. The heat energy supplied by the oven 9′ propagates along the bar 3′mainly in the form of light radiation. By means of a liquid film 17,this light radiation is transmitted from the bar 3′ to the ring 15′which diffuses it into the surrounding medium. The heat energy which isthus trapped and diffused by the silica ring 15′ is therefore nottransmitted to the blind chuck 5′, thereby contributing to reducing itsheating and to preventing any risk of it jamming. It is therefore nolonger necessary to use a blind chuck that is made of refractory steel.

[0037] The above-described variants apply in like manner to apparatus ofthe invention for drawing a glass fiber from a preform.

[0038] The invention is not limited to-the embodiments described above.

[0039] In particular, the space between the diffuser means and thepreform need not be occupied by a liquid film but can be occupied by theambient medium. Under such circumstances, it is preferable to ensurethat the diffuser means fits on the preform with small tolerance.

[0040] In another alternative, the liquid film could be replaced by anindex-matching gel or by appropriate running in.

1. Apparatus for processing an elongate workpiece of optical vitreousmaterial, the apparatus comprising: means for holding said workpieceduring said processing; and heater means supplying said workpiece withheat energy required for said processing, wherein diffuser means areplaced around at least one end of said workpiece to trap and diffuselight radiation generated in said workpiece by said heater means. 2.Apparatus according to claim 1 , wherein said diffuser means has arefractive index close to that of said workpiece.
 3. Apparatus accordingto claim 2 , wherein said diffuser means are made of a vitreousmaterial.
 4. Apparatus according to claim 1 , wherein a liquid film isinserted between said diffuser means and said workpiece.
 5. Apparatusaccording to claim 4 , wherein said liquid film has a refractive indexclose to that of said diffuser means and of said workpiece.
 6. Apparatusaccording to claim 4 , wherein said liquid film is inserted in contactwith a polished surface of said diffuser means.
 7. Apparatus accordingto claim 1 , wherein said diffuser means possesses frosted surfaces. 8.Apparatus according to claim 1 , wherein said diffuser means are heldfixed relative to said workpiece by gaskets.
 9. Apparatus according toclaim 8 , wherein said gaskets are lip gaskets.
 10. Apparatus accordingto claim 1 , wherein a blower blows air over said diffuser means. 11.Apparatus according to claim 1 , wherein said workpiece is a tube formanufacturing an optical fiber preform.
 12. Apparatus according to claim1 , wherein said workpiece is an optical fiber preform.